As a lens for use in a device such as an optical scanner of a laser printer, a lens including a lens portion having two opposite oblong lens surfaces (e.g., fθ lens) is known in the art. Such a lens may be formed by pouring a molten resin material through a gate into a mold which has a cavity contoured to form the shape of the lens and of which a temperature has been adjusted at a predetermined temperature, to fill the cavity with the molten resin material, thereafter letting the mold cool to solidify the material, and removing the solidified casting (molded lens product) from the mold.
The long-length lens such as an fθ lens has a cross-sectional area (i.e., an area of cross section taken along a plane perpendicular to a direction of a length of the lens) varying in accordance with the shape of the lens. To be more specific, for example, the fθ lens is designed to have a cross-sectional area decreasing gradually from a center toward each end in a direction of a length of the fθ lens. As a result, when a resin material is poured into the mold, a difference arises in flowability between a portion closer to a center in the direction of the length of the fθ lens and a portion closer to each end in the direction of the length of the fθ lens.
As the variations of the cross-sectional area along the length of the fθ lens are greater, a difference in thickness between the portion closer to the center in the direction of the length of the fθ lens and the portion closer to each end in the direction of the length of the fθ lens is greater accordingly, and thus, a difference in time required for the material to become solid between the thinnest portion and the thickest portion becomes greater.
The greater the difference in flowability or in time required for the material to become solid, the more likely a defect would develop in the molded product.
There is a need to provide a lens that is molded by injecting a resin material into a mold wherein a likelihood of the molded product suffering a defect is diminished.